This invention relates to ultrasonic harmonic imaging systems and, in particular, to the elimination of flash artifacts from ultrasonic harmonic images.
Ultrasonic imaging systems can now produce diagnostic images using the higher harmonics of the fundamental transmit frequency. Harmonic components in echo signals can arise from two sources: harmonic contrast agents and tissue harmonic distortion. Examples of imaging techniques utilizing these harmonic sources can be found in U.S. Pat. Nos. 5,833,613 and 5,879,303. In both cases the received echoes will contain both large amplitude fundamental frequency signal components and relatively lower amplitude harmonic (nonlinear) components. Consequently, when it is desired to produce an image using only the harmonic components, a method must be employed to separate the harmonic and fundamental frequency components of the echo signal.
Two approaches have been used to separate these components. One is to filter the echo signal so as to pass the harmonic components while attenuating the fundamental components. A high pass or band pass filter can perform this function. The other approach is a multi-pulse signal processing technique known as pulse inversion. Pulse inversion harmonic separation was originally developed for imaging ultrasound contrast agents, but is equally applicable when imaging tissue in the absence of contrast agents. See, for instance, U.S. Pat. Nos. 5,706,819 and 5,951,478. Pulse inversion is a multiple transmit pulse scheme whereby two or more pulses of differently modulated phase, amplitude, and/or polarity are successively transmitted to the same target. Preferably the transmitted pulses are of opposite phase or polarity. The received r.f. echoes from the differently modulated pulses are then summed. In a perfectly stationary target, this summation causes total cancellation of the fundamental signal, resulting in a purely second harmonic signal arising from the non-linearities in the target. In some cases, images of tissue made using pulse inversion harmonic imaging are preferred over conventional (i.e. single pulse) harmonic imaging because of improved speckle suppression and better spatial resolution.
However, when tissue motion occurs between the times that the two pulses are transmitted, incomplete cancellation of the fundamental signal occurs. Since the fundamental component in an echo signal is typically 30 dB larger than the second harmonic component, imperfect cancellation results in a high amplitude fundamental signal dominating the second harmonic signal in scanlines transmitted through sections of moving tissue. In a two dimensional pulse inversion image, the motion induced fundamental component presents itself as a bright flashing of the image in regions where tissue motion occurs, or while the scanhead is moved around in real-time scanning. This flash artifact is rather distracting and is particularly a problem in obstetrics where the fetus is often moving. This artifact is also a problem in abdominal imaging when breathing motion and movement of the scanhead in searching for the desired structures causes this rapid variation in image brightness.
Various approaches have been presented in an effort to eliminate this motion-induced flash artifact. One method involves using a transmit pulse scheme of more than two pulses to better reject the fundamental received echoes from a moving targets. See D. Hope Simpson, C. T. Chin, and P. N. Burns, xe2x80x9cPulse Inversion Doppler: A New Method for Detecting Nonlinear Echoes from Microbubble Contrast Agentsxe2x80x9d IEEE Trans. Ultrason. Ferrorelect., Freq. Contr., Vol. 46, No. 2, pp. 372-382 (1999). The disadvantage to such a multiple pulse scheme is a further reduction in frame rate. If, for instance, three pulses are transmitted for each scanline direction, then the frame rate is concomitantly reduced by ⅔ compared to the standard dual-pulse pulse inversion harmonic image. Since pulse inversion harmonic imaging already reduces the frame rate by a factor of two relative to normal imaging, a further reduction in frame rate is not desirable, especially in abdominal and obstetrical imaging.
In accordance with the principles of the present invention, a method and apparatus are presented for eliminating this harmonic flash artifact. Two or more echo signals from a target are combined to separate harmonic signal components and flash artifacts resulting from relative motion between the tissue being imaged and the transducer. The present inventors have noted that the motion artifact is a large, relatively narrow band signal that occurs at the fundamental frequency. A bandpass or lowpass filter is employed to extract the artifact from the harmonic signal. The extracted artifact is then subtracted from the separated harmonic signal, leaving a harmonic signal substantially free of the motion artifact.